IC-chap 10

Shriver/ Atkins: Inorganic Chemistry

CHAPTER 10: HydrogenCHAPTER 10: Hydrogen


PART A: THE ESSENTIALSPART A: THE ESSENTIALS10.1 The element10.1 The element

Lewis base: hydride ion, HLewis base: hydride ion, H--

Lewis acid: proton, HLewis acid: proton, H++

(a) The atom and its ions(a) The atom and its ions

Key points:Key points: The proton, HThe proton, H++, is always found in combination with a , is always found in combination with a Lewis base and is highly polarizing; the hydride ion, HLewis base and is highly polarizing; the hydride ion, H --, is highly , is highly polarizable.polarizable.

11H: protium H: protium 22H: D, deuterium, 16/100,000 H: D, deuterium, 16/100,000 33H: T, tritium, 1/10H: T, tritium, 1/102121

HH--: two electrons are bound by just one proton : two electrons are bound by just one proton polarizable polarizable


Some of the processes that contribute to the biological hydrogen cycle in a freshwater environment.


(b) Properties and reactions(b) Properties and reactions

Key points:Key points: Hydrogen has unique atomic properties that place it in Hydrogen has unique atomic properties that place it in a special position in the periodic table. Dihydrogen is an inert a special position in the periodic table. Dihydrogen is an inert molecule and its reactions require a catalyst or initiation by molecule and its reactions require a catalyst or initiation by radicals.radicals.

Hydrogen may be found in a metallic state, such as the core of Hydrogen may be found in a metallic state, such as the core of Jupiter.Jupiter.

H—H Bond length: 74 pmH—H Bond length: 74 pm

Electric discharge HElectric discharge H22 H, H H, H++, H, H22++, H, H33

++ hydrogen plasma

Bond enthalpy Bond enthalpy homolytic dissociation Hhomolytic dissociation H22(g) (g) H(g) + H(g) ∆H = 436 kJ/mol H(g) + H(g) ∆H = 436 kJ/mol

heterolytic dissociation Hheterolytic dissociation H22(g) (g) H H++(g) + H(g) + H--(g) ∆H = 1675 kJ/mol(g) ∆H = 1675 kJ/mol

Explosive reaction: 2 HExplosive reaction: 2 H22(g) + O(g) + O22(g) (g) 2 H 2 H22O(g) ∆H = -242 kJ/mol O(g) ∆H = -242 kJ/mol

http://www.youtube.com/watch?v=HnuBoLpf43Y


10.2 Simple compounds10.2 Simple compounds(a) Classification of binary compounds(a) Classification of binary compounds

Key points:Key points: Compounds formed between hydrogen and other Compounds formed between hydrogen and other elements vary in their nature ad stability. In combination with elements vary in their nature ad stability. In combination with metals, hydrogen is often regarded as a hydride; hydrogen metals, hydrogen is often regarded as a hydride; hydrogen compounds with elements of similar electronegativity have low compounds with elements of similar electronegativity have low polarity.polarity.

1. 1. Molecular hydridesMolecular hydrides: individual, discrete molecules formed with : individual, discrete molecules formed with p-block elements. E-H bond: covalent bond Ex: CHp-block elements. E-H bond: covalent bond Ex: CH44, NH, NH33, ,

HH22OO


2. 2. Saline hydridesSaline hydrides: ionic hydrides, formed with the most : ionic hydrides, formed with the most electropositive elements. Ex: LiH, CaHelectropositive elements. Ex: LiH, CaH22 containing H containing H-- ions. ions.

3. 3. Metallic hydridesMetallic hydrides: non-stoichiometric, electrically conducting : non-stoichiometric, electrically conducting solids with a metallic luster. Ex: NaBHsolids with a metallic luster. Ex: NaBH44, LiAlH, LiAlH44

Shriver/ Atkins: Dihydrogen as a FuelDihydrogen as a Fuel

CHCH33OH(g) + HOH(g) + H22O(g) COO(g) CO22(g) + 3 H(g) + 3 H22(g) ∆H = 49 kJ/mol (g) ∆H = 49 kJ/mol

CHCH33OH(g) + ½ OOH(g) + ½ O22(g) CO(g) CO22(g) + 2 H(g) + 2 H22(g) ∆H = -155 kJ/mol (g) ∆H = -155 kJ/mol

Cu/ZnOCu/ZnO

PdPd

Reaction at temperature of 200-350 oC



(b) Thermodynamic consideration(b) Thermodynamic consideration

Key points:Key points: In the s and p blocks, strengths of E-H bonds In the s and p blocks, strengths of E-H bonds decreases down each group. In the d block, strengths of E-H decreases down each group. In the d block, strengths of E-H bonds increase down each group.bonds increase down each group.


H—H is the strongest single H—H is the strongest single homonuclear bond homonuclear bond

a compound to be exergonic and a compound to be exergonic and stable with respect to its elementsstable with respect to its elements

E—H should be stronger than HE—H should be stronger than H—H—H

The weak bond formed by heavier The weak bond formed by heavier p-block elements p-block elements poor overlap between the poor overlap between the relatively compact H 1s orbital and relatively compact H 1s orbital and the more diffuse s and p orbitals of the more diffuse s and p orbitals of their atoms. their atoms.


(c) Reactions of binary compounds(c) Reactions of binary compounds

Key points:Key points: The reactions of binary compounds of hydrogen The reactions of binary compounds of hydrogen fall into three classes depending on the polarity of the E—fall into three classes depending on the polarity of the E—H bond.H bond.


PART B: THE DETAILPART B: THE DETAIL10.3 Nuclear properties10.3 Nuclear properties

Key points:Key points: The three hydrogen isotopes H, D, T have large The three hydrogen isotopes H, D, T have large differences in their atomic masses and different nuclear spins.differences in their atomic masses and different nuclear spins.

ββ decay decay (high-speed e-)

synthesissynthesis

Nuclear fusionNuclear fusion

heated to > 100 MKheated to > 100 MK


10.4 Production of dihydrogen10.4 Production of dihydrogen(a) Small-scale preparation(a) Small-scale preparation

2 Al + 2 OH2 Al + 2 OH-- + 6 H + 6 H22O O 2 Al(OH) 2 Al(OH)44--(aq) + 3 H(aq) + 3 H22(g)(g)

Si + 2 OHSi + 2 OH-- + H + H22O O SiO SiO332-2-(aq) + 2 H(aq) + 2 H22(g)(g)

Zn + 2 HZn + 2 H33OO++ Zn Zn2+2+(aq) + H(aq) + H22(g) + 2 H(g) + 2 H22OO

CaHCaH22(s) + 2 H(s) + 2 H22O O Ca(OH) Ca(OH)22(s) + 2 H(s) + 2 H22(g)(g)


CHCH44(g) + H(g) + H22O(g) O(g) CO(g) + 3 H CO(g) + 3 H22(g) ∆H = 206.2 kJ/mol(g) ∆H = 206.2 kJ/mol

C(s) + HC(s) + H22O(g) O(g) CO(g) + H CO(g) + H22(g) ∆H = 131.4 kJ/mol (g) ∆H = 131.4 kJ/mol at 1000 at 1000 ooCC

CO(g) + HCO(g) + H22O(g) O(g) CO CO22(g) + H(g) + H22(g) ∆H = -41.2 kJ/mol (g) ∆H = -41.2 kJ/mol

C(s) + 2 HC(s) + 2 H22O(g) O(g) CO CO22(g) + 2 H(g) + 2 H22(g) ∆H = 90.2 kJ/mol(g) ∆H = 90.2 kJ/mol

(b) Production from fossil sources(b) Production from fossil sources

Key points:Key points: Most HMost H22 from industry is produced by high- from industry is produced by high-

temperature reaction of Htemperature reaction of H22O with CHO with CH44 or a similar reaction or a similar reaction

with coke.with coke.


HH22O(l) O(l) H H22(g) + ½ O(g) + ½ O22(g) E = -1.23 V, ∆G = 237 kJ/mol(g) E = -1.23 V, ∆G = 237 kJ/mol

(c) Production from renewable sources(c) Production from renewable sources

Key points:Key points: Production of HProduction of H22 by electrolysis of water is costly by electrolysis of water is costly

and viable only in areas where electricity is cheap or if it is and viable only in areas where electricity is cheap or if it is a byproduct of an economically important process. a byproduct of an economically important process.

An electrolysis cell for HAn electrolysis cell for H22 production using Ni anodes and Fe cathodes production using Ni anodes and Fe cathodes

connected in series.connected in series.


10.5 Reactions of dihydrogen10.5 Reactions of dihydrogen

Key points:Key points:

Molecular hydrogen is activated by homolytic or heterolytic Molecular hydrogen is activated by homolytic or heterolytic dissociation on a metal or metal oxide surface or by dissociation on a metal or metal oxide surface or by coordination to a d-block metal.coordination to a d-block metal.

Reactions of hydrogen with OReactions of hydrogen with O22 and halogens involve a and halogens involve a

radical chain mechanism.radical chain mechanism.


(a) Homolytic dissociation(a) Homolytic dissociation

CO(g) + 2 H2(g) CH3OH(g)

(b) Heterolytic dissociation(b) Heterolytic dissociation

Catalyst: Cu/ZnO/AlCatalyst: Cu/ZnO/Al22OO33


(c) Radical chain reactions(c) Radical chain reactions

Initiation, by heat or lightInitiation, by heat or light

BrBr22 Br• + Br• Br• + Br•

PropagationPropagation Br• + HBr• + H22 HBr + H• HBr + H•

H• + BrH• + Br22 HBr + Br• HBr + Br•

Termination Termination H• + H• H• + H• H H22

Br• + Br• Br• + Br• Br Br22

H• + Br• H• + Br• HBr HBr

2 H2 H22(g) + O(g) + O22(g) (g) 2 H 2 H22O(g) ∆H = -242 kJ/molO(g) ∆H = -242 kJ/mol

Shriver/ Atkins: BOX 10.3BOX 10.3 Hydrogen from Solar Energy Hydrogen from Solar Energy

2 Cu(s) + 2 HCl(g) H2 Cu(s) + 2 HCl(g) H22(g) + 2 CuCl(s)(g) + 2 CuCl(s)

4 CuCl(s) 2 Cu(s) + 2 CuCl4 CuCl(s) 2 Cu(s) + 2 CuCl22(s)(s)

2 CuCl2 CuCl22(s) + H(s) + H22O CuO Cu22OClOCl22(s) + 2 HCl(g) (s) + 2 HCl(g)

CHCH22OClOCl22(s) 2 CuCl(s) + ½ O(s) 2 CuCl(s) + ½ O22(g) (g)

425 425 ooCC

325 325 ooCC

electrolysiselectrolysis

550 550 ooCC

(photosensitiser)(photosensitiser)

Dye-Sensitized Solar Cells (DSSCs) Dye-Sensitized Solar Cells (DSSCs)


Dye-Sensitized Solar Cell (DSSCs)Dye-Sensitized Solar Cell (DSSCs)


10.6 Compounds of hydrogen10.6 Compounds of hydrogen(a)(a) Molecular hydridesMolecular hydrides(i)(i) Nomenclature and classificationNomenclature and classificationKey points:Key points: Molecular compounds of hydrogen are classified Molecular compounds of hydrogen are classified

as electron-rich, electron-precise, or electron-deficient. as electron-rich, electron-precise, or electron-deficient.

Electron-precise: Electron-precise: all valence electrons of the central atom all valence electrons of the central atom are engaged in bonds. (CHare engaged in bonds. (CH44, CH, CH33CHCH33, SiH, SiH44, GeH, GeH44))

Electron-rich: Electron-rich: there are more electron pairs on the central there are more electron pairs on the central atom than are needed for bond formation. (lone pairs on atom than are needed for bond formation. (lone pairs on the central atom; NHthe central atom; NH33, H, H22O, HO, H22S)S)

Electron-deficient: Electron-deficient: there are too few there are too few electrons to be able to write a Lewis electrons to be able to write a Lewis structure for the molecule. (Bstructure for the molecule. (B22HH66

3-center, 2-eletron bonds)3-center, 2-eletron bonds)



(ii) Reactions of molecular hydrides(ii) Reactions of molecular hydridesHomolytic Homolytic dissociation occurs for the hydrogen compounds dissociation occurs for the hydrogen compounds

of some p-block elements of some p-block elements RR33SnH + R’X SnH + R’X R’H + R R’H + R33SnX RF < RCl < RBr < RISnX RF < RCl < RBr < RI

((RR33Sn• & H•)Sn• & H•)

Thermal decompositionThermal decomposition AsHAsH33(g) As(s) + 3/2 H(g) As(s) + 3/2 H22 ∆H = - 66.4 kJ/mol ∆H = - 66.4 kJ/mol

OthersOthers HH22O(g) ½ OO(g) ½ O22(g) + H(g) + H22 ∆H = 242 kJ/mol ∆H = 242 kJ/mol

AlHAlH44

-- + 4 RCHO + 4 RCHO Al(OCH Al(OCH22R)R)44--

Al(OH)Al(OH)44-- + 4 RCH + 4 RCH22OHOH

4 H4 H22OO

250-300 250-300 ooCC

2200 2200 ooCC


EXAMPLE 10.1EXAMPLE 10.1 Determining which hydrogen atoms in a molecule are the most acidic

Phosphorous acid, H3PO3, is a diprotic acid and is more helpfully written as OP(H)(OH)2. Explain why the H atom bound to P is much less protonic than the two H atoms bound to O ?

Ans: P-H bond enthalpy: 321 kJ/mol

O-H bond enthalpy: 464 kJ/mol

but O in more electronegative than P O-HH is more protonic


(iii) Hydrogen bonding(iii) Hydrogen bondingKey points:Key points: Compounds and functionalities containing H Compounds and functionalities containing H

atoms attached to electronegative elements with at least atoms attached to electronegative elements with at least one lone pair often associate through hydrogen bonds.one lone pair often associate through hydrogen bonds.

An An E-H E-H is highly polar: is highly polar: --E—HE—H+ + •••• •••• --E—HE—H++ •••••••• --E—HE—H++ hydrogen bondhydrogen bond


Figure 10.6 Normal boiling points of p-block binary hydrogen compounds.Figure 10.6 Normal boiling points of p-block binary hydrogen compounds.

b.p. /m.p. ↑ b.p. /m.p. ↑ H bond ↑ H bond ↑


Figure 10.7 The structure of ice. Figure 10.7 The structure of ice. The structure shows all possible The structure shows all possible atom positions, but only half are atom positions, but only half are actually occupied.actually occupied.

Figure 10.8 Base pairing in DNA. Figure 10.8 Base pairing in DNA. Cytosine recognizes guanine through Cytosine recognizes guanine through the formation of three hydrogen bonds.the formation of three hydrogen bonds.


Cl-H-Cl: linear but not symmetricalCl-H-Cl: linear but not symmetrical F-H-F : not linear but symmetricalF-H-F : not linear but symmetrical

Figure 10.9 The variation of the potential energy with the position of the proton Figure 10.9 The variation of the potential energy with the position of the proton between two atoms in a hydrogen bond.between two atoms in a hydrogen bond.


Figure 10.11 The orientation of lone pairs as indicated by VSEPR theory Figure 10.11 The orientation of lone pairs as indicated by VSEPR theory compared with the orientation of HF in the gas-phase H-bonded complex.compared with the orientation of HF in the gas-phase H-bonded complex.


Figure 10.12 The cages of water molecules in clathrate hydrates; in this case Figure 10.12 The cages of water molecules in clathrate hydrates; in this case XeXe44(CCl(CCl44))88(H(H22O)O)6868..

Shriver/ Atkins: Inorganic Chemistry Methane clathrate hydrate Methane clathrate hydrate

Ionic compound


(b) Saline hydrides(b) Saline hydrides

Electrolysis of molten-salt solution2 H- (melt) H2(g) + 2 e-

Reaction with water (dangerous!!) NaH(s) + H2O NaOH(aq) + H2(g)

1. Metathesis: 4 LiH(s) + SiCl4 4 LiCl(s) + SiH4(g)

2. Addition to a Lewis acid: NaH(s) + B(C2H5)3 Na[HB(C2H5)3]

3. Reaction with a proton source: NaH(s) + CH3OH NaOCH3(s) + H2(g)

Shriver/ Atkins: BOX 10.4 H2 Storage Materials

LiLi33N(s) + HN(s) + H22(g) Li(g) Li22NH(s) + LiH(s) ∆H = 148 kJ/mol NH(s) + LiH(s) ∆H = 148 kJ/mol

at 298 K at 298 K

LiLi22NH(s) + HNH(s) + H22(g) LiNH(g) LiNH22(s) + LiH(s) ∆H = 45 kJ/mol (s) + LiH(s) ∆H = 45 kJ/mol

at 298 K at 298 K

3 bar H3 bar H22, 210 , 210 ooCC

Figure B10.4 Structure relationship between LiFigure B10.4 Structure relationship between Li22NH and LiNHNH and LiNH22

vacuum, > 320 vacuum, > 320 ooCC

3 bar H3 bar H22, 255 , 255 ooCC

vacuum, < 200 vacuum, < 200 ooCC


(C) Metallic hydrides(C) Metallic hydrides

Key points:Key points: No stable binary metal hydrides are known for No stable binary metal hydrides are known for the metals in Groups 7 to 9; metallic hydrides have the metals in Groups 7 to 9; metallic hydrides have metallic conductivity and in many the hydrogen is very metallic conductivity and in many the hydrogen is very mobile. mobile.

Most metal hydrides: - metallic luster, electrically conducting, less dense than the

parent metal, brittle - variable composition (ZrH1.3 ~ ZrH1.75) - CeH2-x is a metallic conductor, CeH3 is an insulator

Ni, Pt – hydrogenation catalysts. Pd – at moderate pressure PdHx, X<1


Figure 10.14 Schematic diagram of a hydrogenFigure 10.14 Schematic diagram of a hydrogen

Pd absorbs up to 900 times its own volume of H2, which is givenoff again on heating. LaNi5 LaNi5H6: greater density of hydrogen than H2(liq)


Figure 10.13 Hydrides formed by d- and f-block elements. The formulae are limiting stoichiometries based on the structure type.


EXAMPLE 10.2EXAMPLE 10.2 Correlating the classification and properties of hydrogen compounds

Classify the compounds PH3, CsH and B2H6 and discuss their probable physical properties. For the molecular compounds specify their subclassification (electron-deficient, electron-precise or electron-rich).

Ans: Cs: group 1 saline hydride insulator & rock-salt structure

P, B: p-block molecules low molar masses &

high volatilities

PH3: lone pair electron-rich

B2H6: electron-deficient


(C) Hydrido and dihydrogen complexes(C) Hydrido and dihydrogen complexes

Key points:Key points: A large number of d-block metals complexes are A large number of d-block metals complexes are known in which the dihydrogen molecule or hydride anion known in which the dihydrogen molecule or hydride anion are ligands. are ligands.

Rh3+(aq) [RhH(NH3)5]2+

[FeI2(CO)4] [Fe(H)2(CO)4]

Zn/NHZn/NH33

NaBHNaBH44



10.7 General methods for synthesis10.7 General methods for synthesis

1. Direct combination (1. Direct combination (hydrogenolysishydrogenolysis)) 2 E + H2 E + H22(g) (g) 2 EH 2 EH

2. 2. ProtonationProtonation of a Brønsted basic anion: of a Brønsted basic anion:

EE-- + H + H22O O EH + OH EH + OH--

3. Reaction of an ionic hydride or hydride donor with a halide3. Reaction of an ionic hydride or hydride donor with a halide ((metathesismetathesis)) EH + EX EH + EX E E++XX-- + EH + EH

NaCl(s) + HNaCl(s) + H22SOSO44(l) (l) NaHSO NaHSO44(s) + HCl(g)(s) + HCl(g)

LiAlHLiAlH44(s) + SiCl(s) + SiCl44(l) (l) LiAlCl LiAlCl44(s) + SiH(s) + SiH44(l)(l)


EXAMPLE 10.3EXAMPLE 10.3 Using hydrogen compounds in synthesis

Suggest a procedure for synthesizing lithium tetraethoxyaluminate, Li[Al(OEt)4], from LiAlH4 and reagents and solvents of your choice.

Ans: AlH4- H- donor: strong Brønsted base than EtO-

LiAlH4 + 4 C2H5OH Li[Al(OEt)4] + 4 H2


EXERCISEEXERCISE

10.210.210.310.310.710.710.810.8

10.1010.1010.1410.1410.2010.20

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